Various circuits have been used widely in lighted displays to control the emissive characteristic of displays such as cathode ray tubes, gas discharge panels, incandescent lighting, and light emitting diodes. Generally speaking, these circuits utilize a variable voltage source controlled by a photo sensor such as a cadmium sulfide photo resistor, to provide a voltage input into the lighting power control circuitry.
The advent of digital processing in Avionics and other precision equipment has allowed the use of micro processor controlled displays for segment configuration (annunciation control) as well as brightness control.
Problems, however, have been observed when utilizing different types of displays in a closely adjacent relationship wherein differences in the circuitry as well as the different characteristics of the respective displays result in varying readability amongst the display units under varying conditions of ambient light.
Additionally, typical display units will have individually controllable dimmer circuits for controlling the brightness level of each box individually, and thereby may develop significant variations among units.
In an avionics environment, it is desirable to maintain high readability under all conditions of ambient lighting amongst all display units. If the response characteristics of the displays are such that varying lighting conditions, for example while flying through broken clouds, results in varying readability characteristics amongst the displays, a potentially hazardeous situation results.